How do I respond to a rejection text?

So there's this girl I went on a date with a while ago but ended up going home and not seeing her for months but then I came back to school. I saw her once by coincidence and a week later reached out to her and asked her to go out for lunch. She responded saying "I'm actually dating someone right now, but I appreciate the thought and offer! I hope classes and life is going well for you :)".
Of course that hurt a lot, but my main question however is what to respond as I feel it is only mature to reply something back. What are some suggestions?

Some ideas I have to include are "Thank you for your honesty", "He's really lucky", "I hope the same for you", etc.
How do I respond to a rejection text?
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